A Promising New Solar Technology With A Troubling Old Toxicity Problem

Perovskite solar cells have stunned researchers with their rapid rise to efficiencies topping 20 percent, making them suddenly competitive with existing technologies. They can be ink-jet printed onto flexible plastic films, suggesting unparalleled ease of manufacture. They can be combined with silicon cells to drive solar efficiencies to new heights.

But they contain lead.

"All perovskite solar cells—high-performing perovskite solar cells—have lead at the B site, and this is pretty significant because lead is toxic," said Adam Slavney, a graduate researcher at Stanford University, in a video Stanford released last week. "Humans have been living around lead and working with lead for a long, long time, and we know now—our experience tells us—that the only level of lead exposure that’s not detrimental to human health is zero—none."

Because of their lead content, perovskite cells could become rooftop perils. If a solar cell cracks when a baseball hits it or a branch falls on it, ensuing rains could sweep the lead onto the roof, into the yard, into the groundwater. Traditional silicon cells contain a small amount of metallic lead that's not readily soluble in water, but perovskites contain an ionic form of lead that's readily soluble.

"I don’t want to oversell this," Slavney said. "I do think that lead perovskites do have a future as a solar technology, but what it does mean is that at every step of the solar cell panel lifecycle, we need to have systems in place that limit lead exposure, and importantly, those systems must be completely failsafe."

Or researchers have to find a substitute for lead, a quest that has come to preoccupy some.

"What we need is to find another element that has the same electronic configuration as lead 2+," Slavney said. "I'm a chemist and usually when I'm confronted with a problem like this I go straight to the periodic table."

On the periodic table, the likeliest candidates are lead's immediate neighbors. But germanium and tin are very sensitive to oxygen, so they degrade in the atmosphere. Thallium, a traditional ingredient in rat poisons, is more toxic than lead. Flerovium is extremely radioactive.

That leaves bismuth.

Bismuth has a charge of +3, and lead has a charge of +2, so to make a bismuth perovskite cell that behaves like a lead perovskite cell, Slavney and other members of Stanford chemistry professor Hema Karunadasa's team built what they call a "double perovskite" that combines +3 bismuth with +1 silver.

It worked. The new perovskite cell not only overcomes the lead problem, it improves on other weaknesses of lead perovskites.

"It has low toxicity because we’ve gotten rid of the lead as well as improved moisture and heat resistance," Slavney said.

But it's not perfect. Stanford's bismuth perovskite is not as good at absorbing available light as the lead version, so it has to be thicker. So the quest to improve it goes on. Other researchers have developed bismuth perovskites, and some have branched into other elements, including antimony and copper.

Safe perovskites are on their way, but it's going to take some time.

Slavney describes the perovskite lead problem— and the research that may solve it—in a presentation to Stanford's Global Climate and Energy Project Symposium:

I've covered the energy and environment beat since 1985, when I discovered my college was discarding radioactive waste in a dumpster. That story ran in the Arizona Republic, and I have chased electrons and pollutants ever since, for dailies in Arizona and California, for alt...